Widespread interest now exists in surface acoustic wave (SAW) devices wherein the wave energy propagates along the surface of a material much as a ripple on a pond. In fact, if viewed microscopically, the surface of the material supporting these waves would resemble such a pond, complete with wave crests and valleys. One of the great advantages of SAW devices results from the fact that the surface waves travel at acoustic (sound) velocities which are much slower than electromagnetic wave propagation. A second equally important advantage is that the acoustic wave energy is continuously accessible along its propagation path. Because the wavelengths are shorter and the energy accessible, components such as delay lines, amplifiers, attenuators, filters and couplers of microminiature construction can be utilized to modify and process electronic signals.
In its simplest form a SAW circuit comprises a source of rf energy, a smooth slab-like element or substrate of material capable of supporting propagating surface waves and a utilization device. Electromechanical transducers are coupled to the substrate to convert the rf energy to surface waves in the material and vice versa. Thus configured, the basic surface wave device is primarily useful as a delay line. Frequently, it is desired to modify the propagating surface wave in a manner such as to enhance the operation of the basic devices and to form new devices having unique properties.
In general, piezoelectric materials are utilized in fabricating the surface wave substrate. With such substrates, the input and output transducers commonly take the form of arrays of thin film conductive interdigitated electrodes which are fabricated on the substrate surface. By properly designing the transducers, it is possible to obtain delay lines with desired characteristics of delay time and frequency response. Because of these properties, such devices are termed delay line filters, and find use in a broad range of communications and radar systems.
Because SAW devices usually have large time delays as compared to LC networks, the temperature coefficient of time delay becomes an important parameter. It is difficult to compensate for temperature effects with simple network techniques. In bandpass filter applications the center frequency, phase shift and time delay are temperature-dependent. In the case of matched filters used in chirp radars, range ambiguities are introduced due to a substrate temperature variation. In phase coded matched filters, the processing gain is seriously degraded as the temperature is varied.
The basic approach taken to date on compensating for delay time changes due to temperature on surface wave substrates has centered on a thin film composite material approach. This approach is characterized by the use of a thin film oxide layer on a surface wave substrate which permits the temperature coefficient of one material to compensate for the temperature coefficient of the other material. Control of the temperature coefficient of delay to small values has been achieved through the use of ZnO film layers on fused quartz and isopaustic glass. This layered film concept has been extended to ZnO film layers on other cuts of quartz and SiO.sub. 2 films on lithium niobate. Specific cuts of quartz and selected glasses have negative temperature coefficients while lithium niobate and zinc oxide possess positive coefficients.
Other approaches have included the butt-joint bonding of ST-quartz to LiNbO.sub. 3 and a resistive film heating arrangement using a phase-lock feedback loop. Placing the surface wave device in a temperature controlled oven is a direct but somewhat clumsy solution to the problem. It would be advantageous to identify a means of temperature compensation which is more simple.
To date, the approaches taken have met with only limited experimental success. It appears against basic structural properties of matter to simultaneously have a high coupling efficiency and low temperature coefficiency in a single material. The best condition obtainable would be that of inducing a temperature stable state in a material of high coupling efficiency and low propagation loss without degrading these properties. It would also be of tremendous benefit if the technique could be adapted to a variety of materials and would be fabricationally simple and reliable.